An immediate cut-off in the electron spectrum below some cutoff energy is unphysical. We have instead assumed a distribution of the form \(N(\gamma ) \propto \gamma ^2\)  for electrons below the critical Lorentz factor \(\gamma _{\rm cr}\) . This low-energy distribution corresponds to the low-energy tail of a relativistic Maxwellian, which is likely to arise if the low energy cut-off is due to the dissipation/thermalisation of jet bulk kinetic energy in the hotspots, as argued by [1]}. This model will produce a more gradual turnover in the radio spectrum than that produced with the classical LEC model which assumes an immediate cutoff in the electron spectrum. We note that a non-uniform magnetic field strength and/or variation in the cutoff energy as a function of position would act to make the turnover even more gradual than our model, and in that sense our model is conservative. The magnetic field strengths for hotspots A and D are assumed to be 160 and 250 \(\mu \) G, respectively, as given by the synchrotron self-Compton modelling of the hotspot X-ray emission [2]}.
